Substrates of the ubiquitin (Ub)-dependent N-end rule pathway include proteins with destabilizing N-terminal residues. A set of amino acids that are destabilizing in a given cell yields a rule, called the N-end rule, which relates the in vivo half-life of a protein to the identity of its N-terminal residue. The essential component of a degradation signal called the N-degron is a destabilizing N-terminal residue of a protein. Variants of the N-end rule pathway are present in all organisms examined, from animals and plants to fungi and prokaryotes (FIG. 1A). In eukaryotes, an N-degron of a protein consists of a destabilizing N-terminal residue and an internal lysine, the latter being the site of formation of a substrate-linked multi-Ub chain. The N-end rule has a hierarchic structure (FIG. 1A). N-terminal Asn and Gln are tertiary destabilizing residues in that they function through their deamidation, by N-terminal amidohydrolases, to yield the secondary destabilizing residues Asp and Glu. The activity of N-terminal Asp and Glu requires their conjugation, by ATE1-encoded Arg-tRNA-protein transferases (R-transferases), to Arg, one of the primary destabilizing residues. The latter are recognized by the E3 (Ub ligase) components of the N-end rule pathway (FIG. 1A). In mammals, the set of destabilizing residues that function through their arginylation contains not only Asp and Glu but also Cys, which is a stabilizing (non-arginylated) residue in the yeast Saccharomyces cerevisiae. The known species of mammalian R-transferase, ATE1-1 and ATE1-2, are produced through alternative splicing of ATE1 pre-mRNA, and have the same substrate specificity as the yeast R-transferase, they arginylate N-terminal Asp or Glu, but cannot arginylate N-terminal Cys.
Angiogenesis is the growth of new blood vessels. Generally in the body, a balance of angiogenesis growth factors and angiogenesis inhibitors keeps this process of blood vessel growth under control. However, a lack or excess of either angiogenesis growth factors or angiogenesis inhibitors can cause undesired growth of blood vessels or a failure to produce blood vessels.